In Long Term Evolution Advanced (LTE-Advanced) systems, multiple component carriers can be aggregated in uplink and downlink, respectively. For example, to provide high data rates, the User Equipment (UE) can receive simultaneous transmissions on multiple downlink component carriers. For one UE, each component carrier is used for transmission of 1 transport block (2 transport blocks for Multiple Input Multiple Output (MIMO) transmission). If the detection of a transport block is successful, the UE sends an Acknowledgement (ACK) on the uplink and if the detection was unsuccessful, a Negative Acknowledgement (NACK) is sent. An ACK may be represented with a bit taking the value ‘1’ and a NACK may be represented with the value ‘0’. Thus, with carrier aggregation, multiple ACK or NACK bits need to be transmitted from one UE in response to the transmitted transport blocks over different component carriers.
Sometimes the notion of a cell is used instead of component carrier, and aggregation could equivalently be for cells. In an aggregation case the User Equipment may be configured with multiple serving cells. A cell may provide both an uplink and a downlink direction of communication and may thus comprise both an uplink and downlink component carrier. A UE can be assumed to have a Primary serving cell (PCell) as well as one or several Secondary serving cells (SCells). A person skilled in the art could equivalently use the terminology of cells instead of component carriers in relation to carrier aggregation.
Before detecting the transport block, a downlink control channel first needs to be detected which contains the downlink assignment information needed to receive the data channel and to decode the transport block. If the UE did not correctly receive the control channel, the UE is not aware of that it is expected to receive any data channel and it does not send any feedback, neither ACK nor NACK in the uplink. This is referred to as discontinuous transmission (DTX). The eNB knows when to expect a NACK or ACK and upon DTX detection, the eNodeB would have to initiate a re-transmission, if it performed a data transmission. One downlink control channel transmission contains the assignments for both transport blocks in a MIMO transmission. Hence, DTX applies to both transport blocks simultaneously for MIMO.
In addition to missing a downlink assignment, the ACK/NACK signalling in the uplink may be erroneous, e.g., a transmitted ACK may be received as a NACK, or a transmitted NACK may be received as an ACK. A NACK-to-ACK error may introduce Hybrid automatic repeat request (HARQ) buffer corruption in the receiver due erroneous combining of several transmissions, since the User Equipment (UE) may expect a retransmission while the basestation (for instance eNodeB in the Long Term Evolution (LTE) communications system) schedules a new packet. An ACK-to-NACK error causes inefficient system operation due to unnecessary retransmissions which the UE is not expecting. It is therefore important to provide robust ACK/NACK signalling. To assure proper system performance, the LTE specifications list requirements on the ACK/NACK error performance.
Channel selection is one method that is feasible for transmission of multiple Acknowledgement (ACK) and Negative Acknowledgement (NACK) bits. The transmission is performed by Quadrature Phase Shift Keying (QPSK) (or other modulation order symbol) modulated sequences and the feedback information is encoded by both the selection of the channel in the form of a sequence and the QPSK constellation point. The channel selection refers to the selection of the sequence and several sequences can be transmitted on the same frequency resource. That is, channels are obtained by Code Division Multiplexing (CDM) of sequences. A UE is typically only transmitting on one channel at a time. Therefore, since only one sequence is selected and transmitted for one User Equipment (UE), channel selection preserves the single-carrier property of the signal which is beneficial for keeping a low Peak to Average Power Ratio (PAPR) of the UE. This feedback principle was used already in Long Term Evolution (LTE) Rel-8 for Time Division Duplex (TDD), where ACK/NACKs from multiple downlink subframes are signalled by one transmission in a single uplink subframe. For LTE-Advanced, channel selection will also be used, but in the context of conveying ACK/NACKs from multiple component carriers, i.e., carrier aggregation. This applies for up to of 4 ACK/NACK bits and also includes the Frequency Division Duplex (FDD) case. Each transport block generally requires one ACK/NACK bit, thus 4 bits may correspond to a configuration of 2 component carriers with MIMO transmission on each.
To encode the ACK/NACK/DTX information, a mapping is needed between the different states of ACK, NACK and DTX, and the channels and QPSK constellation points. These mapping tables exist for channel selection for TDD in Rel-8, but new mappings are needed for carrier aggregation in LTE-Advanced because of different combinations of ACK/NACK/DTX will need to be encoded and that the availability and reservation of the channels will differ.
Transmission of ACK/NACKs for carrier aggregation by means of channel selection requires a codebook where for each valid combination of ACK/NACK/DTX, one channel and one constellation point should be assigned and there is thus a need for such a scheme for a mobile radio communications system that employ carrier aggregation.